The present invention relates to the catalytic cracking of heavy hydrocarbons in general and more specifically to a method for efficiently cracking heavy hydrocarbon feed stocks, such as resids, which contain high levels of metals such as nickel and/or vanadium.
Catalytic cracking has been used for more than forty years to convert heavy charge stocks to lighter more valuable materials. There are two general types of catalytic cracking processes--fluidized bed and moving bed processes.
Fluidized catalytic cracking (FCC) is by far the most popular process. In it a charge stock contacts a stream of hot, regenerated catalyst. The charge stock cracks and in so doing it deposits coke on the catalyst. The catalyst is stripped of strippable hydrocarbons (usually with steam) and then regenerated with an oxygen containing gas (usually air).
In moving bed catalytic cracking or Thermofor catalytic cracking (TCC) the process flow is much the same in that catalyst contacts oil and cracks it and becomes deactivated with coke. The main distinction between the two processes is that the FCC process uses fine particles of catalyst, usually in the 10-80 micron range while the TCC or moving bed process uses beads or extrudates typically of 1/8-1/16 inch diameter.
There has been increasing interest in cracking heavier charge stocks. Many refiners now add resid or residual fractions to the FCC feed. These materials heretofore had been fed to a coker or visbreaker or used to make asphalt for roads. As gasoline, diesel and similar distillable products are more valuable than resid, there is much economic incentive to convert the residual fraction of the crude oil into lighter products. Unfortunately there are several problems with cracking resids. One of the major ones is that the metals content of the crude oil tends to be high in the residual fractions. These metals are poisons on the FCC or TCC catalyst. Any nickel present in the feed is deposited on the FCC catalyst and adds a hydrogenation/dehydrogenation function to the catalyst. Hydrogenation/dehydrogenation components are essential in many refinery processes (such as hydrotreating, and hydrocracking) but cannot be tolerated in catalytic cracking. The downstream processing steps cannot accomodate the huge volumes of hydrogen, methane and other light gases produced during cracking when a hydrogentation/dehydrogenation component is present on the catalyst.
Vanadium is undesirable as a catalytic component, but causes an additional problem. The vanadium seems to attack the zeolite based FCC or TCC catalyst. Vanadium may act as a cancer to destroy the zeolite structure. The problems of vanadium attack on FCC catalyst are not completely understood, however it is understood that vanadium is a severe problem and that it should be removed from the feed or removed from the catalyst.
A discussion of the mechanism of vanadium poisoning is reported in Vanadium Posioning of Cracking Catalysts, Wormsbecher et al, Journal of Catalysis 100, 130-137 (1986). This reference suggests that the high temperature, steam laden atmosphere of FCC regenerators converts V.sub.2 O.sub.5 into 1-10 ppm, of H.sub.3 VO.sub.4. Addition of a basic alkaline earth solid, such as MgO or CaO, is proposed to neutralize this vanadic acid.
Some attempts have been made at adjusting FCC (or TCC) operation to accomodate higher metals levels. The Phillip's metals passivation process is a popular way of passivating the metal contaminants, particularly Ni present in the feed. Typically, antimony and tin compounds are added to the feed to passivate the nickel and vanadium, respectively. In practicing metals passivation the metals still accumulate on the catalyst, however their bad effects are moderated by the addition of antimony or other materials.
Another approach has been to remove some of the catalyst from the catalytic cracking unit and send it to a metals recovery unit, and perhaps recycle the catalyst back to the FCC unit.
Plank in U.S. Pat. No. 2,668,798, was one of the first to address the problem of catalyst deactivation. He studied the poisioning of an amorphous cracking catalyst with nickel. The examples used steam and acid treatment of spent catalyst to remove nickel. This treatment was thought to be suitable for removal of other metal contaminants such as copper, iron, vandium, and the like.
The DEMET process provides a multistage procedure for treating catalytic cracking catalyst to remove much of the metals content and restore much of the original activity of the catalyst. A catalyst demetallization process is discussed more fully in U.S. Pat. No. 4,686,197, and EP No. 0 252 659 Al.
Another approach has been to modify the FCC catalyst, or provide an additive catalyst, which can trap the nickel/vanadium components in the feed. This material, sometimes referred to as a "getter", or "scavenger" is one which preferentially adsorbs metals from the feed, so that they do not remain in the feed to be adsorbed by the FCC catalyst. Such a scavenger was disclosed by Wormsbecher et al, in a paper presented at the Ninth North American Catalyst Society Meeting, Houston Tex., Mar. 18-21 ,1985.
Most refiners also practice careful cracking catalyst inventory control when cracking heavy feeds. Catalyst removal rates of 1-2 weight percent a day are typical in FCC processing to maintain catalyst activity. When heavy, metals laden feed is used catalyst addition rates may double, quadruple or go even higher to maintain the level of metals in the FCC catalyst inventory.
Unfortunately, all of these solutions to the problems of too much metal in the feed have their drawbacks. In general they allow the problem to be created and then try to cope with it later. Thus, most of the solutions allow the catalyst to be poisoned and then try to cope with it by metals passivation, dumping catalyst and replacing it more frequently, or removing a slip stream of the circulating catalyst and cleaning it up and returning it to the unit.
Addition of "getter" materials, which have an affinity for Ni, V and other impurities (including coke precursors) to the catalyst is helpful, but the getters do not function as efficiently as desired. These greater additives have a size similar to that of FCC catalyst (to remain in the unit) so their surface area is similar to that of the FCC catalyst. The FCC catalyst is always present in excess, and the FCC catalyst competes with the additives for the metal in the feed. Unless large concentrations of getter additive are present (which dilute the cracking catalyst) a lot of metal is till deposited on the cracking catalyst. The metal captured by the getter additive remains in the unit, and may form vanadic acid. The metals that accumulate on the greater may transfer metal to the FCC catalyst. The metals on the getters do as much harm as that on the FCC catalyst.
We realized that the most efficient way to deal with the problem of too much metals in the feed was to attack the problem at its source, i.e., to intercept the metals before they could be deposited on the catalyst. We found existing feed demetallation technology inadequate--the low cost approaches such as guard beds did not work well and more effective processes (expanded be hydrotreaters) cost too much. Both guard bed and expanded bed technology will be briefly reviewed.
Guard bed treating the feed upstream of the FCC or TCC process has not been too successful because at the relatively low temperatures of the hydrocarbon feed it is difficult to remove all of the metals from the FCC feed. The metals can be removed to some extent by treatment with ion exchange resins, or acids or bases, but none of these treatments are able to remove enough metal to be completely satisfactory. All allow a significant amount of metal to escape through the feed pretreatment step to then contaminate the catalytic cracking catalyst.
Expanded bed, high pressure hydrotreating processes such as H-Oil and LC Fining will remove metals and upgrade heavy oil feeds, but the high pressures (1000-2000 psig) and high hydrogen consumptions associated with such units make them impractical for use upstream of a catalytic cracking unit.
We have now discovered a way to efficiently remove metals from the feed, by using an extremely fine additive which is dispersed with the feed. The extremely fine additive will, in the presence of hot regenerated catalyst, be an extremely efficient getter material for recovering metals from the feed because the getter has a high surface area and is almost perfectly distributed with the feed. The getter material will be quickly removed from the FCC or TCC unit via these mechanisms because its small particle size will allow it to be blown out the unit as "fines" either with the hydrocarbon product or with the flue gas.